remove board_init_irq():  On all in-kernel boards it is sufficient to
initialize board interrupts in an arch_initcall by using the default
linux irq functions.

Some small irqmap.c files have been folded into board_setup files.

Run-tested on DB1200; compile-tested on all other affected boards.
Signed-off-by: NManuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>